Carburizing method

ABSTRACT

A process is disclosed for selectively carburizing (tailored carburizing) a steel gear or similar ferrous metal object. The process of the present invention includes two similar but separate methods. The first method includes the steps of: first, masking the roots of the gear with an appropriate masking material, and conducting a first carburizing operation of the gear. Then, the masking material is removed from the gear roots and a second carburizing operation of the gear is conducted. The resulting carburized gear will have a lighter case with a lower carbon content in the root areas of the gear. The second method includes the steps of: masking the root areas of the gear with a coating material which would inhibit carbon diffusion in the root areas but not completely stop it. Then, carburizing the gear. Finally, the inhibiting agent is removed. The resulting carburized gear will have a lighter case with a lower carbon content in the root areas of the gear.

This application claims the benefit under 35 U.S.C. § 119 of U.S. provisional application Ser. No. 60/684,528 filed May 26, 2005, which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to systems and methods of carburizing. More specifically, the invention relates to systems and methods for carburizing ferrous metal articles to cause different carbon content at different locations.

2. Description of Related Art

Carburizing is a widely known region or surface hardening process. The process involves diffusing carbon into a low carbon steel alloy to form a high carbon steel region or surface. The diffused carbon normally reacts with alloys of the steel to enhance the hardness of the steel surface. The diffused carbon normally reacts with alloys of the steel to enhance the hardness of the steel surface. Carburizing may result in a component that has a high surface hardness and a softer core. Carburizing, therefore, may be especially useful for treating high wear components such as gears and shafts. Enhanced surface hardness may provide suitable resistance to frictional and impact wear without sacrificing desirable properties of the bulk material.

Carburizing is generally performed by heating parts in a furnace in the presence of an atmosphere capable of diffusing carbon into the surface of the parts. The amount or percent of carbon is controlled by the composition of the atmosphere. The depth of diffusion of carbon into the steel may be controlled by the temperature of the component and time of exposure to an environment containing carbon. Most often, a part to be carburized is heated in a furnace to a desired carburizing temperature in an environment including carbon. After carburizing, the work may be either slow-cooled for later quench hardening, or quenched directly into various gaseous or liquid quenches. Because carbon has been diffused into the surface this area of the part hardens to a very high hardness while the core hardens to an intermediate hardness level. Normally the entire surface of the part is carburized although ceramic paints can be used to prevent carburization in certain areas if desired. The carburized case is relatively uniform in composition and in depth around the part.

The need exists for useful method to tailor the case depth and carbon content of the case differently for various portions of a part rather than to take what the carburizing process provides.

SUMMARY OF THE INVENTION

The invention is a process for selectively carburizing (tailored carburizing) ferrous metal article, such as a gear. The process of the present invention includes two similar but separate methods. The first method includes the steps of: first, masking an region or area of the article with an appropriate masking material, and conducting a first carburizing operation of the article. Then, the masking material is removed from the article and a second carburizing operation of the article is conducted. The resulting carburized article will have a lighter case with a lower carbon content in the masked region or area of the article. The second method includes the steps of: masking an area of the article with a coating material which would inhibit carbon diffusion in the masked region or area but not completely stop it. Then, the article is carburized. Finally, the inhibiting agent is removed. The resulting carburized article will have a lighter case with a lower carbon content in the masked region of the article.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic of the first step of masking the root area for the first methodology of the present carburizing invention.

FIG. 1B is a schematic of the second step of carburizing the exposed area for the first methodology of the present carburizing invention.

FIG. 1C is a schematic of the third step of removing mask included in the first methodology of the present carburizing invention.

FIG. 1D is a schematic of the fourth step of re-carburizing the tooth to include the root area as set forth in the first methodology of the present carburizing invention.

FIG. 2A is a schematic of the first step of applying an inhibiting agent to the root area for the second methodology of the present carburizing invention.

FIG. 2B is a schematic of the second step of carburizing the entire tooth including the area having the inhibiting agent according to the second methodology of the present carburizing invention.

FIG. 2C is a schematic of the third step of removing the inhibiting agent according to the second methodology of the present carburizing invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

It has been shown that the performance of a carburized part is dependent on the percent of carbon in the case as well as the depth of the case. For example, in bending of a gear tooth a lighter or shallower case is better for strength and impact. However, for contact strength and durability on the face of a gear tooth, a deeper case is preferred. Likewise for maximum bending fatigue life, lower carbon content in the case is better. However, for maximum contact fatigue life a higher percentage of carbon is better.

This invention is to tailor the case depth and carbon content of the case differently for various portions of a part rather than to take what the carburizing process provides. This is intended to maximize the performance of a part in different areas. For example on a gear tooth a lighter case depth and lower carbon content could be used in the root area to improve bending properties while a deeper case and higher carbon content could be used on the tooth face to improve contact properties.

One method of tailoring the carburized case in different areas of the part would be separate carburizing operations. The tooth face could be carburized to a higher carbon content and deeper depth with the root area masked off.

FIG. 1A is a schematic of the first step of masking the root area for the first methodology of the present carburizing invention. As shown in FIG. 1A, only the root area of the gear teeth 10 receives the masking agent 20. For this purpose, this covering surface is then covered with an agent preventing such carbonization, such as, for example, the trade product available under the name Condursal®. Such agent is a high temperature resistant mixture of sand and oil. The agent can be any material that is removable after heat treatment and prevents ingress of the carburizing gas to the welding zone. Such agent can be selected from the group consisting of Condursal®, high temperature microcrystalline wax, high temperature resistant lacquer, carbon stop-off paint, ceramic paint and other agents known to those of skill in the art. With reference to FIG. 1B, the second step of this first method includes carburizing the exposed area 30 of the gear teeth 10 to 1.2 mm case depth and surface carbon of 0.90%. Next, the masking agent 20 is removed from the root 12 (see FIG. 1C), and the gear teeth are re-carburized including the root area as shown in FIG. 1D to include the root areas at 0.4 mm case depth and surface carbon of 0.60% for the root areas.

A modified carburizing operation could follow the steps of FIGS. 1A-1D, but it would put a lighter case with lower carbon content in the root area. The first carburizing operation would preferably be at a carbon content higher than actually desired so that the diffusion that takes place during the second operation would bring the carbon content down to the desired level.

Another potential method for tailoring the carburized case would be to use a coating which would inhibit carbon diffusion in certain areas of the part rather than completely stop it or mask the area. This second preferred process would follow the steps illustrated by FIGS. 2A-2C.

FIG. 2A is a schematic of the first step of applying an inhibiting agent to the root area for the second methodology of the present carburizing invention. The inhibiting agent would block some carbonization, but not all. A nickel plating material is the preferred inhibiting agent; however, other materials that suitably partially block carbonization will satisfy the parameters of this invention. As shown in FIG. 2B, the gear teeth 10 a including the area having the inhibiting agent 20 a is carburized according to the second methodology of the present carburizing invention. As a result, the exposed area 10 a and treated area 20 a is carburized to different degrees. FIG. 2C is a schematic of the third step of removing the inhibiting agent with the resulting carburization at the tooth 30 resulting in a case depth of approximately 1.2 mm and a surface carbon of approximately 0.90%, while resulting in a case depth of approximately 0.4 mm and a surface carbon of approximately 0.60% at the root area 40.

As described above, the invention is a process for selectively carburizing (tailored carburizing) a ferrous metal article, such as a steel gear. By way of example, the process of the present invention includes two similar but separate methods:

1) the first method includes the steps of: first, masking the roots of the gear with an appropriate masking material, and conducting a first carburizing operation of the gear. Then, the masking material is removed from the gear roots and a second carburizing operation of the gear is conducted. The resulting carburized gear will have a lighter case with a lower carbon content in the root areas of the gear;

2) the second method includes the steps of: masking the root areas of the gear with a coating material which would inhibit carbon diffusion in the root areas but not completely stop it. Then, carburizing the gear. The resulting carburized gear will have a lighter case with a lower carbon content in the root areas of the gear.

While the foregoing method has been shown and described with reference to several preferred embodiments, it will be understood by those of skill in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the present invention. Applicant has chosen to use a gear as the exemplary article; however, the processes of the present invention may be applied to a variety of ferrous metal articles. 

1. A method for selectively carburizing a ferrous metal article, said method comprising the steps of: providing said ferrous metal article having at least one first region and at least one second region; covering said at least one second region of said ferrous metal article with a layer of masking material provided to substantially isolate said at least one second region of said ferrous metal article from a carburizing atmosphere; carburizing said ferrous metal article in the carburizing atmosphere with said at least one second region thereof covered with said masking material; removing said layer of said masking material; and carburizing said ferrous metal article to form a carburized article in the carburizing atmosphere, wherein at least one of a case depth and a region carbon content of said at least one first region of said ferrous metal article in a final form is higher than the respective case depth and the region carbon content of said at least one second region of said ferrous metal article.
 2. The method according to claim 1, wherein said at least one second region is disposed at a root area between gear teeth.
 3. The method according to claim 1, wherein said at least one first region is disposed at adjacent portions of the gear surrounding the root area of the gear teeth.
 4. The method according to claim 1, wherein said case depth of said at least one second region is approximately 0.4 mm.
 5. The method according to claim 1, wherein said region carbon content of said at least one second region is approximately 0.60%.
 6. The method according to claim 1, wherein said case depth of said at least one first region is approximately 1.2 mm.
 7. The method according to claim 1, wherein said region carbon content of said at least one first region is approximately 0.90%.
 8. A method for selectively carburizing a ferrous metal article, said method comprising the steps of: providing said ferrous metal article having at least one first region and at least one second region; covering said at least one second region of said ferrous metal article with a layer of a coating material provided to partially inhibit carbon diffusion in said at least one second region of said ferrous metal article in a carburizing atmosphere; carburizing said ferrous metal article with said at least one second region thereof covered with said coating material; selectively removing said coating material; wherein at least one of a case depth and a region carbon content of said at least one first region of said ferrous metal article in a final form is higher respectively than the case depth and carbon region content of said at least one second region of said ferrous metal article.
 9. The method according to claim 8, wherein said at least one second region is disposed at a root area between gear teeth.
 10. The method according to claim 8, wherein said at least one first region is disposed at adjacent portions of the gear surrounding the root area of the gear teeth.
 11. The method according to claim 8, wherein said case depth of said at least one second region is approximately 0.4 mm.
 12. The method according to claim 8, wherein said region carbon content of said at least one second region is approximately 0.60%.
 13. The method according to claim 8, wherein said case depth of said at least one first region is approximately 1.2 mm.
 14. The method according to claim 8, wherein said region carbon content of said at least one first region is approximately 0.90%. 